Karachi's defensive mangrove barrier faces triple threat

Thick mangroves have long protected Karachi, southern Pakistan's sprawling metropolis, from battering by the Arabian Sea, but pollution, badly managed irrigation and years of illegal logging have left this natural barrier in a parlous state.

Experts fear that loss of the natural barrier formed by the mangroves could put the city of nearly 20 million people at greater risk from violent storms and even tsunamis.

Close to Karachi, the mighty Indus river ends its long journey from the Himalayas in the sea.

"If we were in another country, the mangrove would be valued and protected."

As well as dissipating the energy of tropical storms when they hit the coast, the mangrove also provides a line of defence in case of tsunamis.

The Arabian and Eurasian tectonic plates meet at the Makran Trench, off the coast, and the boundary has the potential to create major earthquakes.

An undersea quake in 1945 generated a tsunami that hit Karachi, killing 4,000 people, and a recent UN simulation suggested the city could be wiped out if a big tremor hit again.

"It is a very important ecosystem... it is the first line of defence against cyclones, strong surges, tsunami and other natural calamities," said marine biologist Khan, who works for the WWF wildlife NGO.

But there is some hope. A drive to replant the mangroves in recent years has seen them slowly regain some of the losses.

"It (plantation) is going very well. There are very few areas in the world where the mangroves cover is increasing and Pakistan is one of them," Khan said.